Signal-processing device for a vehicle having an abs unit, vehicle, signal-processing method for a vehicle, computer programme and control unit

ABSTRACT

The invention relates to a signal-processing device ( 402 ) for a vehicle having an ABS unit ( 404 ) and multiple wheels, each of which is assigned a sensor (S 1,  S 2,  S 3,  S 4 ) for wheel signal generation. The signal-processing device ( 402 ) is designed to detect ( 602 ) a failure of a wheel signal, to form ( 604 ) a substitute signal for the failed wheel signal using the wheel signal of at least one sensor assigned to a wheel that is not affected by the failure, and to supply ( 606 ) the substitute signal to the ABS unit ( 404 ). The invention also relates to an ABS system ( 400 ) having the signal-processing device ( 402 ) and an ABS unit ( 404 ), a vehicle having the ABS system ( 400 ), a signal-processing method for a vehicle having an ABS unit ( 404 ), a computer programme having computer code for carrying out the signal-processing method, as well as a control unit containing the computer programme.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a national stage of International Application No.PCT/EP2018/085406, filed Dec. 18, 2018, the disclosure of which isincorporated herein by reference in its entirety, and which claimedpriority to German Patent Application No. 102017012130.3, filed Dec. 28,2017, the disclosure of which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The present disclosure relates generally to the field of vehicle brakesystems. Specifically, aspects relating to the operation of such a brakesystem in the case of a failure of a wheel signal are described.

BACKGROUND

Known hydraulic vehicle brake systems, which are embodied as abrake-by-wire (BBW) system or are equipped with an electric brake boost(EBB) system comprise an actuator which can be actuated electrically andin the service brake mode generates a hydraulic pressure at the wheelbrakes of the motor vehicle or boosts a hydraulic pressure generated bythe driver. For this purpose, a deceleration of the vehicle which isrequested by the driver at a brake pedal is sensed by a sensor andconverted into an actuation signal for the actuator.

Such brake systems generally also comprise a master cylinder which canbe activated mechanically by means of the brake pedal and via whichhydraulic fluid can also be delivered to the wheel brakes. The mastercylinder which can be activated by means of the brake pedal providesredundancy, indispensable for reasons of operational safety, withrespect to the BBW or EBB system which can be actuated electrically.

Modern brake systems also comprise a vehicle movement dynamics controlsystem (also referred to as electronic stability control, ESC) whichcomprises, for example, one or more functions such as a traction controlsystem (TSC), an anti-lock brake system (ABS) or an electronic stabilityprogram (ESP). There are requirements also to configure the vehiclemovement dynamics control system in a redundant fashion. In other words,in the event of a loss of functioning of the vehicle movement dynamicscontrol system at least rudimentary vehicle movement dynamics control isstill to be possible in order to be able to maintain the stability ofthe vehicle or the deceleration capability at least partially.

Brake systems for autonomous or partially autonomous vehicles also haveto be configured in a redundant fashion, especially since the driver isnot necessarily located in the vehicle (e.g. in a remote controlledparking (RCP) operation) or cannot intervene directly in the operationof the vehicle. The vehicle movement dynamics control system and, herein particular the ABS, becomes highly significant in this context. Forexample, stringent requirements are made of the ABS with respect to itsavailability. In a conventional vehicle, the ABS can in fact be easilydeactivated in the event of a fault and the driver can be alerted tothis deactivation, in order to encourage him to maintain a safer drivingstyle. In contrast, during autonomous or partially autonomous drivingthe vehicle system remains fully responsible over a long time period oreven permanently.

A frequent fault situation which brings about deactivation of the ABS ina conventional vehicle is the failure of a wheel signal which is used bythe ABS for slip detection and slip control on the corresponding vehiclewheel. In order to prevent this fault situation, the corresponding wheelsensor system (if appropriate including the feed lines) can beconfigured in a redundant fashion. However, such redundancy entails highcosts. As an alternative to this, in the case of a wheel signal failure,for example for a front wheel, a closed-loop control operation at thewheels of the front axle can be deactivated, and the wheels of the rearaxle could be concentrated on in order to prevent at least oversteering.However, the stability limitations which result from this approach arenot acceptable in many cases, for example during autonomous or partiallyautonomous driving.

BRIEF SUMMARY

The present disclosure is based on the object of specifying technicalsolutions which are less susceptible to the failure of a wheel signal.

According to a first aspect, a signal-processing device is specified fora vehicle having an ABS unit and a plurality of wheels which are eachassigned a sensor for generating wheel signals. The device is designedto detect a failure of a wheel signal, to form a substitute signal forthe failed wheel signal using the wheel signal of at least one sensorwhich is assigned to a wheel which is not affected by the failure, andto feed the substitute signal to the ABS unit.

The device can be part of an. ABS. For example, the device can beinstalled in an ABS control unit or be implemented in some other way(e.g. using a processor unit and software).

According to one variant, the device is also designed to make aselection from wheel signals of those sensors which are assigned towheels which are not affected by the failure. In this case, thesubstitute signal can correspond to the selected wheel signal or be atleast decisively based thereon. Accordingly it is possible to form thesubstitute signal decisively on the basis of the selected wheel signal,but additionally one or more further wheel signals which are presentand/or other types of sensor signals can be taken into account in theformation of the substitute signal. The substitute signal is then based,for example, decisively on the selected wheel signal if a deviationbetween the substitute signal and the selected wheel signal is less than20%, in particular less than 10% or less than 5%.

The selection from the wheel signals can be made in accordance with adetected split μ situation. In a split μ situation, differentcoefficients of friction of the underlying surface are present for afirst wheel on a first side of the vehicle and a second vehicle wheel ona second opposite side of the vehicle. The two vehicle wheels which lieopposite one another and which have different coefficients of frictionof the underlying surface can be assigned, in particular, to the samevehicle axle.

In one variant, the failed wheel signal is assigned to a rear wheel.According to this variant, the device is also designed to carry outsplit μ detection or split μ plausibility checking on the basis of wheelsignals of sensors which are assigned to front wheels. According toanother variant, the failed wheel is assigned to a first front wheel.According to this variant, the device is also designed to carry outsplit μ detection or split μ plausibility checking on the basis of wheelsignals of sensors which are assigned to a second front wheel which isnot affected by the failure and to a rear wheel which is diagonallyopposite the second front wheel. The plausibility checking of a split μsituation by the device can be used for verifying whether the previouslydetected split μ situation is still present.

The abovementioned selection from wheel signals of sensors which areassigned to wheels which are not affected by the failure can compriseselecting the wheel signal from that wheel which lies opposite the wheelvehicle side which is affected by the failure. Such a selection strategycan be implemented, in particular, when a split μ situation is notpresent or is not detected.

Furthermore, when a split μ situation is detected, the selection can bemade in accordance with whether the wheel signal has failed for a wheelof a side with a high coefficient of friction or a side with a lowcoefficient of friction. In the case of a failure of a wheel signal fora first rear wheel on a side with a high coefficient of friction, thewheel signal can be selected for a second rear wheel on a side with alow coefficient of friction. In the case of a failure of a wheel signalfor a rear wheel on a side with a low coefficient of friction, the wheelsignal for a front wheel on the side with the low coefficient offriction can be selected. In the case of a failure of a wheel signal fora front wheel on a side with a high coefficient of friction, the wheelsignal for a rear wheel on the side with the high coefficient offriction can be selected. In the case of a wheel signal for a firstfront wheel on a side with a low coefficient of friction, the wheelsignal for a second front wheel on the side with a low coefficient offriction can be selected.

An anti-lock brake system which comprises the signal-processing devicepresented above and an ABS unit is also specified. The signal-processingdevice and the ABS unit can be accommodated together in an ABS controlunit.

According to one variant, the ABS unit comprises an assigned wheelsignal input for each sensor (and therefore for each wheel). In thiscase, the signal-processing device can be designed to feed thesubstitute signal to that wheel signal input which is assigned to thefailed wheel signal. The wheel signal inputs can be implementedphysically in the form of hardware and/or logically in the form ofsoftware.

The signal-processing device can also be designed to generate a failuresignal which indicates the wheel affected by the failure. The failuresignal can indicate, for example, that the failure relates to aright-hand rear wheel, to a left-hand rear wheel, to a right-hand frontwheel or to a left-hand front wheel. The anti-lock brake device can havean output for the failure signal. The signal-processing device can feedthe failure signal to the ABS unit via this output.

The ABS unit can be designed to detect, on the basis of the at least onesubstitute signal (and if appropriate one or more of the wheel signalswhich are still present), a need for an anti-lock brake controloperation at the wheel which is affected by the failure and/or to carryout an anti-lock brake operation at the wheel which is affected by thefailure.

When a split μ situation is present, the ABS unit can be designed tocarry out a select low closed-loop control operation of the rear axle inthe case of a failure of a wheel signal for a rear wheel. In addition oras an alternative, the ABS unit can be designed to activate anindividual closed-loop control operation in order to bring about minimumdeceleration at the rear axle in the case of failure of a wheel signalfor a front wheel.

The failed wheel signal can be assigned to a rear wheel. In this case,the ABS system can be designed to carry out split μ detection or split μplausibility checking on the basis of wheel signals of sensors which areassigned to front wheels. If the failed wheel signal is assigned to afirst front wheel, the system can be designed to carry out split μdetection or split μ plausibility checking on the basis of wheel signalsof sensors which are assigned to a second front wheel which is notaffected by the failure or to a rear wheel which lies diagonallyopposite the second front wheel.

In one variant, the ABS unit is designed to generate an indicationsignal which indicates a split μ situation. The indication signal canindicate, for example for a specific vehicle wheel or a specific side ofthe vehicle (on the left-hand side/on the right-hand side) a specificcoefficient of friction of the underlying surface or generally anindication of a high coefficient of friction of the underlying surfaceor a low coefficient of friction of the underlying surface. In thiscase, the signal-processing device can have an input for the indicationsignal in order to be able to receive the indication signal from the ABSunit.

The system can also be designed to calculate a target brake pressure fora wheel at which an increase in brake pressure is necessary. Theincrease in brake pressure may be necessary in relation to a normalservice braking operation or an ABS-assisted service braking operationor in relation to something else. The calculation of the target brakepressure can be carried out in accordance with the wheel signal which isobtained for this wheel and in accordance with a determination as towhether this wheel is affected by a wheel signal failure.

The ABS unit can be designed to calculate, for the wheel affected by thefailure, a slip threshold for the use of an anti-lock brake controloperation. The slip threshold can be defined, for example, as a maximumpermitted difference between the vehicle speed and the wheel speed,estimated on the basis of the substitute signal, of the wheel which isaffected by the failure. The slip threshold which is calculated for thewheel affected by the failure may be lower than if the wheel were notaffected by the failure. In this way, the slip threshold can be reducedfor a specific wheel if a wheel signal failure has been detected forthis wheel and the slip control is based at least essentially on thesubstitute signal.

A vehicle which comprises the anti-lock brake control system describedhere is also specified. The vehicle can be designed for autonomous orpartially autonomous driving.

A further aspect of the present disclosure relates to asignal-processing method for a vehicle having an ABS unit and aplurality of wheels which are each assigned a sensor for generatingwheel signals. The method comprises detecting a failure of a wheelsignal, forming a substitute signal for the failed wheel signal usingthe wheel signal of at least one sensor which is assigned to a wheelwhich is not affected by the failure, and feeding the substitute signalto the ABS unit.

Another aspect of the present disclosure relates to a device for avehicle having a plurality of wheels which are each assigned a sensorfor generating wheel signals. The unit is designed to determine whetherone of the wheels is affected by a failure of the corresponding wheelsignal, to acquire wheel signals which are assigned to the wheels,wherein for a wheel affected by a wheel signal failure the correspondingwheel signal is acquired in the form of a substitute signal, and tocalculate a target brake pressure for a wheel at which an increase inbrake pressure is necessary, in accordance with the wheel signal whichis acquired for this wheel, and in accordance with the determination asto whether this wheel is affected by a wheel signal failure.

The unit may be part of an ABS. In addition or as an alternative, theunit can also be part of an EBB or BBW system. Therefore, the unit canbe installed in an ABS control unit and/or in a control unit for an EBBor BBW system, or can be implemented in some other way (e.g. using aprocessor unit and software).

The substitute signal may have been generated in any desired way. Forexample, the substitute signal may occur by estimating on the basis ofsensor signals which are available in some other way and which do notnecessarily have to comprise wheel signals.

The increase in the brake pressure may be necessary in relation to anormal service braking operation, for example if the driver activatesthe brake pedal. The increase in brake pressure can also occur for anABS-assisted braking operation or in some other way (e.g. in the case ofan emergency braking operation).

The device can also be designed to reduce the target brake pressure incases in which the wheel at which the increase in brake pressure isnecessary is affected by the wheel signal failure. Specifically, thetarget brake pressure which is calculated on the basis of the wheelsignal which is obtained for this wheel (that is to say the substitutesignal) can be given a lower setting than if the wheel were not affectedby a wheel signal failure. In this context, the target brake pressurecan be applied starting from the point when the predefined minimumvehicle deceleration or a predefined minimum brake pressure is reachedat the wheel affected by the failure.

The device can also be designed to form the substitute signal using thewheel signal of at least one sensor which is assigned to a wheel whichis not affected by the failure. This substitute signal can correspond,in particular as stated above, to a wheel signal (or be based decisivelythereon) which is assigned to a comparison wheel which is not affectedby the failure. In this case, the target brake pressure at the wheelwhich is affected by the failure can be calculated in such a way that itis lower than a wheel brake pressure which is calculated for thecomparison wheel.

In addition or as an alternative, a predefined pressure difference canbe maintained between the wheel affected by the failure and thecomparison wheel (in particular if they are assigned to the same vehicleaxle). In this context, absolute value of the pressure difference candepend on whether a signal of at least one further sensor of a vehicledynamics control system is available. This further sensor is differentfrom the sensors for generating wheel signals and can be designed, forexample, to sense a longitudinal acceleration, a transverseacceleration, a steering angle, a main cylinder pressure or a yaw rate.

The device can also be designed to compensate pulling of the vehicle toone side as a result of the pressure difference (in particular at wheelson the same axle). This can occur, in particular, as a result of thesetting of a brake pressure difference on opposing wheels of a vehicleaxle which is not affected by the wheel signal failure.

Furthermore, the device can be designed to determine a slip thresholdstarting from which an anti-lock brake control operation starts at awheel. The determination of the slip threshold can occur in accordancewith the determination as to whether this wheel is affected by the wheelsignal failure. In this context, the slip threshold for a wheel which isaffected by the wheel signal failure can be set lower than the slipthreshold which is for a wheel on the same axle and is not affected bythe wheel signal failure.

Likewise, a vehicle is specified which comprises the unit presentedhere. The vehicle can be designed, in particular, for autonomous orpartially autonomous driving.

A further aspect relates to a method for a vehicle having a plurality ofwheels which are each assigned a sensor for generating wheel signals.The method comprises determining whether one of the wheels is affectedby a failure of the corresponding wheel signal, acquiring wheel signalswhich are assigned to the wheels, wherein, for a wheel which is affectedby a wheel signal failure, the corresponding wheel signal is obtained inthe form of a substitute signal, and calculating a target brake pressurefor a wheel at which an increase in brake pressure is necessary, inaccordance with the wheel signal which is acquired for this wheel and inaccordance with the determination as to whether this wheel is affectedby a wheel signal failure.

The methods presented here can also comprise method steps whichcorrespond to the functions of the devices and units which are describedhere.

A computer program with program code for carrying out the methodsdescribed here when the computer program is run on a processor unit isalso specified.

A control unit or system comprised of a plurality of control unitscomprising at least one processor unit and at least one memory is alsospecified, wherein the at least one memory contains program code forcarrying out the method presented here when said program code runs onthe at least one processor unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects, details and advantages of the present disclosure emergefrom the following description of exemplary embodiments with referenceto the figures, of which:

FIG. 1 shows an exemplary embodiment of a vehicle brake system;

FIG. 2 shows an exemplary embodiment of a control unit system for thebrake system according to FIG. 1;

FIG. 3 shows an exemplary embodiment of an ABS for the brake systemaccording to FIG. 1;

FIG. 4 shows the ABS according to FIG. 3 in a state without a wheelsignal failure;

FIG. 5 shows the ABS according to FIG. 3 in a state with a wheel signalfailure;

FIG. 6 shows a flow diagram of an exemplary embodiment of a method foroperating the ABS according to FIG. 4;

FIGS. 7A-D show various exemplary embodiments for generating asubstitute signal in the case of a wheel signal failure without a splitμ situation;

FIGS. 8A-D show various exemplary embodiments for generating asubstitute signal in the case of a wheel signal failure in a split μsituation;

FIG. 9 shows a flow diagram of an exemplary embodiment of a furthermethod for operating the ABS according to FIG. 4; and

FIG. 10 shows a schematic diagram which illustrates the application of abrake pressure difference in relation to the method according to FIG. 9.

DETAILED DESCRIPTION

FIG. 1 shows the hydraulic circuit diagram of an exemplary embodiment ofa hydraulic vehicle brake system 100. It is to be noted that theteachings which are presented here and which relate to the failure of awheel signal are not limited to a hydraulic brake system with the designillustrated in FIG. 1, but rather it is only intended to give anexemplary explanation here on the basis of this brake system.

The brake system 100 according to FIG. 1 comprises an assembly 110 whichcan be coupled to a brake pedal (not shown) and has the purpose ofgenerating hydraulic pressure and a hydraulic control assembly 120 (alsoreferred to as a hydraulic control unit, HCU) with two separate brakecircuits I. and II. The brake system 100 also comprises four wheelbrakes. Two of the four wheel brakes 130 are assigned to the brakecircuit I., while the two other wheel brakes 130 are assigned to thebrake circuit II. The assignment of the wheel brakes 130 to the wheelbrakes I. and II. is carried out here according to a diagonal divisionsuch that the wheel brakes 130A and 130B on the right-hand rear vehiclewheel (RR) and respectively on the left-hand front wheel (FL) areassigned to the brake circuit I., while the wheel brakes 130C and 130Don the left-hand rear wheel (RL) and respectively on the right-handfront wheel (FR) are assigned to the brake circuit II. The wheel brakes130 can alternatively also be distributed to the wheel brakes I. and II.on an axle basis.

The brake system 100 also comprises in the present exemplary embodimentan optional electric parking brake (EPB) with two electromechanicalactuators 140A, 140B which can be electrically actuated separately fromone another. In FIG. 1, the actuators 140A, 140B are each indicated onlyin the form of an electric motor. Of course, the actuators 140A, 140Ecan comprise further components, such as for example a transmission, viawhich the actuators 140A, 140B act, for example, on wheel brakecylinders.

The two actuators 140A, 1406 are assigned differently to the four wheelbrakes 130. Specifically, the actuator 140A is assigned to the wheelbrake 130A of the right-hand rear wheel (RR), while the actuator 140B isassigned to the wheel brake 130C of the left-hand rear wheel (RL). Inother variants, the two actuators 140A, 140B can also be assigned to thewheel brakes 130B, 130D of the right-hand front wheel (FR) andrespectively of the left-hand front wheel (FL).

The assembly 110 for generating hydraulic pressure comprises a mastercylinder 110A and can be operated according to the EBB principle and/orthe BBW principle. This means that installed in the assembly 110 is anactuator which can be actuated electrically and is in the form of ahydraulic pressure generator 110B which is designed to boost or togenerate a hydraulic pressure for at least one of the two brake circuitsI. and II. This hydraulic pressure generator 110E comprises an electricmotor which acts directly or indirectly on the master cylinder 110A inorder to generate hydraulic pressure via a mechanical transmission. Anindirect effect can occur, for example, in a hydraulic fashion (forexample if the transmission acts on a plunger arrangement whose outputis coupled hydraulically to an input of the master cylinder 110A).

The HCU 120 comprises a vehicle movement dynamics control system (alsoreferred to as an ESC system), which is embodied with two circuits inthe present example and has the purpose of carrying out closed-loopcontrol interventions at the wheel brakes 130. In other exemplaryembodiments, the vehicle movement dynamics control system can also beembodied in a known fashion with a single circuit.

Specifically, the two-circuit vehicle movement dynamics control systemaccording to FIG. 1 comprises a first hydraulic pressure generator 160,which can be actuated electrically, in the first brake circuit I., and asecond hydraulic pressure generator 170, which can be actuatedelectrically, in the second brake circuit II. Each of the two hydraulicpressure generators 160, 170 comprises an electric motor 160A, 170B aswell as a pump 160B, 170B which can be actuated by the electric motor160A, 170B. Each of the two pumps 160B, 170B can be embodied as amulti-piston pump, as a gear pump or as another type of pump. Each pump160B, 170B shuts off counter to its delivery direction, as isillustrated by means of the shutoff valves at the output and the inputof the pumps 160B, 170B. Since the rotational speed of each of theelectric motors 160A, 170A can be adjusted, the delivery quantity ofeach of the pumps 160B, 170B can also be adjusted by correspondinglyactuating the assigned electric motor 160A, 170A.

The two electric motors 160A, 170A—and therefore the two hydraulicpressure generators 160, 170—can be actuated independently of oneanother. This means that each of the two hydraulic pressure generators160 and 170 can increase a hydraulic pressure independently of the otherhydraulic pressure generator 170 or 160 in the respective brake circuitI. and II. This redundancy is an optional feature of the brake system100, but is advantageous in terms of technical safety considerations.

The brake system 100 operates by means of a hydraulic fluid which ispartially stored in three reservoirs 110C, 190, 200. While the reservoir110C is a pressureless reservoir which forms part of the assembly 110,the two other reservoirs 190, 200 are each installed as pressureaccumulators (e.g. as low pressure accumulators, LPA) in one of the twobrake circuits I., II. The two hydraulic pressure generators 160 and 170are each able to suck in hydraulic fluid from the assigned reservoir 190or 200 or from the central reservoir 110C.

The reservoir 110C has a larger capacity than each of the two reservoirs190, 200. However, the volume of the hydraulic fluid which isrespectively stored in the two reservoirs 190, 200 is at leastsufficient to be able to bring a motor vehicle safely to a standstilleven when a brake pressure control operation is necessary at one or moreof the wheel brakes 130 (e.g. in the case of ABS-assisted emergencybraking).

The brake circuit I. comprises a hydraulic pressure sensor 180A which isarranged on the input side of the brake circuit I., in the region of itsinterface with the assembly 110. The signal of the hydraulic pressuresensor 180A can be evaluated in conjunction with actuation of thehydraulic pressure generator 110B, which is installed in the assembly110, and/or of the hydraulic pressure generator 160 which is installedin the brake circuit I. The evaluation and actuation are carried out bymeans of a control unit system 300 which is only shown schematically inFIG. 1. In a corresponding way, a further hydraulic pressure sensor 180Bis installed in the brake circuit II.

Furthermore, each wheel is assigned precisely one wheel sensor S(denoted by S1 to S4 in FIG. 1). The wheel sensors S are designed togenerate a wheel signal indicating the rotational speed or velocity ofthe corresponding wheel. Wheel-related slip detection and slip controlcan also be carried out by the ABS on the basis of the wheel signals.

As shown in FIG. 1, the two brake circuits I. and II. are of identicaldesign with respect to the components installed therein and thearrangement of these components. For this reason, only the design andthe method of functioning of the first brake circuit I. are explained inmore detail below.

In the brake circuit I., a multiplicity of valves are provided which canbe activated by electromagnets and assume the basic positionsillustrated in FIG. 1 when in the non-activated, that is to sayelectrically non-actuated state. In these basic positions, the valvesconnect the assembly 110, in particular the master cylinder 110A, to thewheel brakes 130. Therefore, even when there is a loss of function (e.g.a failure) of the energy supply and an associated failure of thehydraulic pressure generator 110B, a hydraulic pressure can still beincreased at the wheel brakes 130 by the driver by means of the brakepedal which acts on the master cylinder 110A. However, in the case of anEBB implementation, this hydraulic pressure is in fact not boosted, orin the case of a BBW implementation mechanical coupling of the brakepedal to the master cylinder 110A occurs (push-through, PT operation).In the BBW operation, the master cylinder 110A is, on the other hand,fluidically decoupled from the brake circuit I. in a known fashion.

The multiplicity of valves comprises two 2/2-way valves 210, 220 whichpermit decoupling of the two wheel brakes 130A and 130B from theassembly 110. Specifically, the valve 210 is provided to uncouple, inthe electrically actuated state, the wheel brakes 130A, 130B from theassembly 110 if a control intervention at at least one of the two wheelbrakes 130A, 130B is carried out by means of the hydraulic pressuregenerator 160. In its electrically actuated state, the valve 220 makesit possible for the hydraulic fluid to be sucked in or continue to besucked in from the reservoir 110C (e.g. in the case of a persistentcontrol intervention, if the reservoir 190 is completely emptied in theprocess). In addition, in this electrically actuated state, a reductionin pressure at the wheel brakes 130A, 130B is possible by making itpossible for hydraulic fluid to flow back from the wheel brakes 130A,130B into the pressureless reservoir 110C.

The hydraulic connection of the wheel brake 130A, 130B to the assembly110 and to the hydraulic pressure generator 160 is determined by four2/2-way valves 230, 240, 250, 260 which assume the basic positionsillustrated in FIG. 1 in the non-activated, that is to say electricallynon-actuated state. This means that the two valves 230 and 260 eachassume their open position while the two valves 240 and 250 each assumetheir closed position. The two valves 230 and 240 form a first valvearrangement which is assigned to the wheel brake 130B, while the twovalves 250 and 260 form a second valve arrangement which is assigned tothe wheel brake 130A.

As explained below, the two valves 210 and 220, the two valvearrangements 230, 240 and respectively 250, 260 and the hydraulicpressure generator 160 are each designed to be actuated for wheel brakepressure control interventions at the respective wheel brake 130A, 130B.The actuation of the two valves 210 and 220, of the two valvearrangements 230, 240 and respectively 250, 260 and of the hydraulicpressure generator 160 within the scope of the control interventions iscarried out by means of the control unit system 300. The control unitsystem 300 implements, for example, the wheel brake pressure controlinterventions of a vehicle movement dynamics control system, wherein thevehicle movement dynamics control system according to the presentdisclosure comprises at least one anti-lock brake control system (ABS).In addition, a traction control system (TCS) and/or an electronicstability program (EPB) and/or a brake pressure control system for anadaptive cruise control (ACC) system can also be included therein.

An anti-lock brake control operation is intended to prevent locking ofthe wheels during a braking operation. This requires the hydraulicpressure in the wheel brakes 130A, 130B to be modified individually inaccordance with the prevailing wheel, slip. As mentioned above, in orderto determine the wheel slip the signals to the wheel sensors S1 to S4are evaluated (more on this later). The ABS pressure modulation iscarried out by adjusting pressure-increasing phases,pressure-maintaining phases and pressure-reducing phases which alternatein a chronological sequence and result from suitable actuation of thevalve arrangements 230, 24O and respectively 250, 260 which are assignedto the two wheel brakes 130B and 130A, and, if appropriate, of thehydraulic pressure generator 160.

During a pressure-increasing phase, the valve arrangements 230, 240 andrespectively 250, 260 each respectively assume their basic position sothat the brake pressure in the wheel brakes 130A, 130B can be increased(as in the case of a BBW braking operation) by means of the hydraulicpressure generator 160. For a pressure-maintaining phase at one of thewheel brakes 130B and 130A, just the valve 230 or respectively 260 isactuated, that is to say is moved into its locking position. Since thevalve 240 or respectively 250 is not actuated, it remains in its closedposition. As a result, the corresponding wheel brake 130B or 130A isdecoupled hydraulically, so that a hydraulic pressure which occurs inthe wheel brake 130B or respectively 130A is kept constant. In apressure-reducing phase, both the valve 230 or respectively 260 and thevalve 240 or respectively 250 is actuated, that is to say the valve 230or respectively 260 is moved into its closed position and the valve 240or respectively 250 is moved into its open position. Therefore,hydraulic fluid can flow away from the wheel brake 130B or respectively130A in the direction of the reservoirs 110C and 190, in order to reducea hydraulic pressure which is present in the wheel brake 103A orrespectively 130B.

Other control interventions in the normal service braking mode occur inan automated fashion and typically independently of an activation of thebrake pedal by the driver. Such automated control operations of thewheel brake pressure occur, for example, in conjunction with a tractioncontrol operation which prevents individual wheels spinning during astarting process by targeted braking, a vehicle movement dynamicscontrol system in the narrower sense, which system adapts the vehiclebehavior in the boundary region to the driver's request and the roadwayconditions through targeted braking of individual wheels, or an adaptivecruise control operation which maintains a distance between the driver'svehicle and a vehicle traveling ahead, inter alia by automatic braking.

When an automatic hydraulic pressure control operation is carried out, ahydraulic pressure can be increased at at least one of the wheel brakes130A or respectively 130B by actuating the hydraulic pressure generator160. In this context, the valve arrangements 230, 240 and respectively250, 260 which are assigned to the wheel brakes 130B, 130A of thehydraulic pressure generator 160 firstly assume their basic positionsillustrated in FIG. 1. Fine adjustment or modulation of the hydraulicpressure can be carried out by corresponding actuation of the hydraulicpressure generator 160 and of the valves 230, 240 and respectively 250,260 which are assigned to the wheel brakes 130B and respectively 130A,as is explained by way of example above in conjunction with the ABScontrol operation.

The hydraulic pressure control is carried out by means of the controlunit system 300, generally in accordance with, on the one hand,parameters which are acquired by sensor and which describe the vehiclebehavior (e.g. wheel speeds of the sensors S1 to S4, yaw rate,transverse acceleration, etc.) and, on the other hand, parameters whichare acquired by sensor (e.g. activation of the brake pedal, steeringwheel angle, etc.) and which describe the driver's request, insofar asthey are present. A deceleration request of the driver can bedetermined, for example, by means of a travel sensor which is coupled tothe brake pedal or to an input element of the master cylinder 110A. Inaddition or as an alternative, the brake pressure which is generated inthe master cylinder 110A by the driver can be used as a measurementvariable which describes the driver's request, said brake pressure thenbeing sensed by means of the sensor 180A (and the corresponding sensor180B assigned to the brake circuit II.), and if appropriate itsplausibility is checked. The deceleration request can also be initiatedby a system for autonomous or partially autonomous driving.

FIG. 2 shows an exemplary embodiment of the control unit system 300 fromFIG. 1. As illustrated in FIG. 2, the control unit system 300 comprisesa first control unit 302 which is designed to actuate the hydraulicpressure generator 160 and the EPB actuator 140A, and a second controlunit 304 which is designed to actuate the hydraulic pressure generator170 and the EPB actuator 140B. As explained in conjunction with FIG. 1,this actuation can take place on the basis of a multiplicity ofmeasurement variables which are acquired by sensor. In another exemplaryembodiment, the two control units 302 and 304 can also be combined toform a single control unit, in particular in a single-circuitconfiguration of the vehicle movement dynamics control system.

In the exemplary embodiment according to FIG. 2, the two control units302 and 304 are embodied as a spatially coherent control unit device306. In this way, the two control units 302 and 304 can be accommodatedin a common housing but comprise separate processors 302A, 304A forprocessing the measurement variables and for actuating the respectivelyassigned components 140A, 160 and respectively 140B, 170 and separatememories 302B, 304B. In order to exchange data, for example in relationto the plausibility checking of measurement variables and/or actuationsignals, the corresponding processors 302A, 304A of the two controlunits 302, 304 are communicatively connected to one another via aprocessor interface 308. The processor interface 308 is embodied in theexemplary embodiment as a serial-parallel interface (SPI).

The control unit system 300 also comprises a third control unit 310which is designed to actuate the hydraulic pressure generator 110Binstalled in the assembly 310, and therefore, in particular the electricmotor of said hydraulic pressure generator 110. Depending on theconfiguration of the brake system 100, this actuation can take placeaccording to the EBB principle or the BBW principle. The control unit310 can form a spatially coherent control unit device with the two othercontrol units 302 and 304 or else can be provided spaced aparttherefrom. In one implementation, a housing of the control unit 310 isintegrated into the assembly 110. In a system for autonomous orpartially autonomous driving, the control unit system 300 can comprise afurther control unit (not illustrated in FIG. 2) which implements thecorresponding functions.

As shown in FIG. 2, in the present exemplary embodiment two parallelelectrical supply systems K30-1 and K30-2 are provided (in otherexemplary embodiments, in particular in a single-circuit configurationof the vehicle movement dynamics control system just a single of thesesupply systems K30-1 and K30-2 could be present). Each of these twosupply systems K30-1 and K30-2 comprises a voltage source (notillustrated) as well as associated voltage supply lines. In theexemplary embodiment according to FIG. 2, the supply system K30-1 isdesigned to supply the EPB actuator 140A and the hydraulic pressuregenerator 160, while the parallel supply system K30-2 is designed tosupply the other EPB actuator 140B and the hydraulic pressure generator170. In another exemplary embodiment, the EPB actuator 140A and thehydraulic pressure generator 160 could additionally (that is to say in aredundant fashion) be capable of being supplied by the supply systemK30-2, and the EPB actuator 140B and the hydraulic pressure generator170 could additionally be capable of being supplied by the supply systemK30-1. In this way, the system redundancy is increased further.

Each of the three control units 302, 304 and 310 (as well as an optionalcontrol unit for autonomous or partially autonomous driving) is suppliedin a redundant fashion both via the supply system K30-1 and via thesupply system K30-2. For this purpose, each of the control units 302,304, 310 can be provided with two separate supply connections which areeach assigned to one of the two supply systems K30-1 or respectivelyK30-2.

As is also illustrated in FIG. 2, two parallel communication systemsBus1 and Bus2 are provided in a redundant fashion and are each embodiedin the exemplary embodiment as a vehicle bus (e.g. according to the CANor LIN standard). The three control units 302, 304 and 310 (as well asan optional control unit for autonomous or partially autonomous driving)can communicate with one another via each of these two communicationsystems Bus1, Bus2. In another exemplary embodiment, just a single bussystem (e.g. Bus1) could be provided.

The wheel sensors S1 to S4 (and if appropriate the further sensors) arealso connected to at least one of the two supply systems K30-1 and K30-2as well as at least one of the two communication systems Bus1 and Bus2.In this way, the control units 302, 304 are supplied with wheel signalsfor the ABS implemented therein (and for possible further ESC functionswhich are implemented therein).

In the exemplary embodiment according to FIG. 2, the actuation of thecomponents 140A, 160 and 140B, 170 is carried out by means of the twocontrol units 302 and 304, and the actuation of the hydraulic pressuregenerator 110B which is installed in the assembly 110 is carried out bymeans of the control unit 310 (or by means of the optional control unitfor autonomous or partially autonomous driving) in such a way that thecorresponding control unit 302, 304, 310 activates or deactivates and,if appropriate, modulates the power supply for the correspondingcomponent (e.g. by means of pulse width modulation). In anotherexemplary embodiment, one or more of these components, in particular theEBP actuators 140A, 140B can be connected to one or both of thecommunication systems Bus1, Bus2. In this case the actuation of thesecomponents by means of the assigned control unit 302, 304, 310 is thencarried out via the corresponding communication system Bus1, Bus2. Inaddition, in this case the corresponding component can be continuouslyconnected to one or both of the supply systems K30-1, K30-2.

FIG. 3 shows an exemplary embodiment of an implementation of an ABS 400,which can be integrated into the control unit system 300 of the brakesystem 100 according to FIG. 1 or a control unit or control unit systemwhich is configured in some other way.

If the control unit system 300 comprises two separate control units 302and 304 with an independent ESC functionality (cf. FIG. 2), each of thecontrol units 302 and 304 can comprise the ABS 400 according to FIG. 3in a redundant fashion. As an alternative, it would also be conceivablethat each of the two control units 302 and 304 implement just one partof the ABS 400 for those two wheels which are assigned to thecorresponding control unit 302, 304. Another type of implementation ofthe ABS 400 or a modified form thereof is conceivable in conjunctionwith the control unit system 300 illustrated in FIG. 2.

As illustrated in FIG. 3, the ABS 400 comprises a signal-processingdevice 402 and an ABS unit 404. The signal-processing device 402comprises four inputs E1 to E4 for wheel signals and four outputs A1 toA4, also for wheel signals. The inputs E1 to E4 of the signal-processingdevice 402 are connected to, in each case, one of the wheel sensors S1to S4 via a communication system (for example via the two parallelcommunication systems Bus1 and Bus2 according to FIG. 2).

In FIG. 3, the four inputs E1 to E4 of the signal-processing device 402are illustrated as four logically separate inputs. Of course, these fourlogic inputs E1 to E4 can be mapped onto a single physical input(connection). This applies in a corresponding way for the four outputsA1 to A4 of the signal-processing device 402.

The four inputs E1 to E4 and the four outputs A1 to A4 of thesignal-processing device 402 are coupled to one another via amultiplexer 406. The multiplexer 406 permits any input El to E4 to becoupled to any of the outputs A1 to A4. Any of the inputs E1 to E4 canalso be coupled to two or more outputs A1 to A4. In one development, themultiplexer 406 is also capable of processing the wheel signals whichare received via the inputs E1 to E4 (for example of mixing them) and ofoutputting one or more signals which have been processed in this way viaone or more of the outputs A1 to A4. For example, the wheel signalswhich are acquired by means of a plurality of the inputs E1 to E4 cantherefore be processed with different weighting to form a new wheelsignal and fed to one or more of the outputs A1 to A4.

It is basically the case that the signal-processing device 402 isdesigned to receive wheel signals via the inputs E1 to E4 and to outputsignals via the outputs A1 to A4. The signals which are output alsoconstitute wheel signals from the point of view of the ABS unit 404,even though they can differ from the wheel signals received by thesignal-processing device 402, owing to the operations of the multiplexer406.

The signal-processing device 406 comprises a further input/outputinterface A/E5, in order to be able to communicate with the ABS unit404. The ABS unit comprises for this purpose a complementaryinput/output interface E/A5.

The ABS unit also comprises four inputs E1 to E4 which can be coupled tothe corresponding outputs A1 to A4 of the signal-processing device 402.Via these inputs E1 to E4, the ABS unit 404 accordingly receives signalswhich, from the point of view of the ABS unit 404, are each assigned toone of the wheels at which the corresponding wheel sensor S1 to S4 isinstalled. In other words, the ABS unit 404 assigns one of the wheelsensors S1 to S4 to each of its inputs E1 to E4.

The ABS unit 404 also comprises an ABS logic 408. The ABS logic 408 isdesigned to subject wheel signals received via the inputs E1 to E4 toABS processing. This ABS processing comprises, for example, calculatinga wheel slip, detecting the exceeding of a slip threshold by a specificwheel and carrying out an ABS control operation at the wheel whichexceeds the slip threshold. This slip control includes calculatingactuation signals for ABS pressure modulation related to one wheel, asis explained with reference to the valve arrangements and hybridpressure generators illustrated in FIG. 1. The corresponding actuationsignals are output by the ABS unit 404 via corresponding outputs A1 toA4.

As has already been explained in relation to the signal-processingdevice 402, the inputs E1 to E4 and the outputs A1 to A4 of the ABS unit404 are logic inputs or logic outputs which can be implemented by meansof one or more physical inputs or outputs.

FIG. 4 shows an operating state of the ABS 400 in a fault-free state ofthe brake system 100. The fault-free state means here that a wheelsignal from each of the wheel sensors S1 to S4 is respectively presentat the corresponding input E1 to E4 of the signal-processing device 402.The wheel signal which is present at the respective input E1 to E4 ispassed on from the multiplexer 406 to the corresponding output A1 to A4of the signal-processing device 402 without further processing and isoutput to the corresponding input E1 to E4 of the ABS unit 404. The ABSlogic 408 processes the wheel signals in a known fashion in order todetect slip and, if necessary, to control slip. If the need to performslip control is detected at one or more of the vehicle wheels,corresponding actuation signals are output via one or more of theoutputs A1 to A4 of the ABS unit 404. The actuation signals which areoutput then bring about ABS pressure modulation at the assigned wheelbrake 130A to 130D.

In the fault-free state of the brake system 100 no communication isnecessary between the signal processing device 402 and the ABS unit 404via the interfaces E/A5.

FIG. 5 illustrates the operation of the ABS 400 in the event of afailure of a wheel signal. Specifically, in the example illustrated inFIG. 5 it is assumed that the wheel signal of the wheel sensor S1 whichis assigned to the left-hand front wheel FL has failed. The failure ofthe wheel signal of the wheel sensor S1 may be attributable to a failureof this sensor S1 itself or can have other reasons (for example theinterruption of a signal-transmission line between the wheel sensor S1and the input E1 of the signal-processing device 402).

It is explained below, with reference to the flowchart 600 according toFIG. 6, how the signal-processing device 402 reacts to the failure ofthe wheel signal of the wheel sensor S1.

In a first step 602, the signal-processing device 402 detects thefailure of the wheel signal at the input E1. At the same time it isdetected that wheel signals from the assigned sensors S2 to S4 continueto be detected at the remaining inputs E2 to E4. The detected failure ofthe wheel signal of the wheel sensor S1 can be communicated to the ABSunit 404 by means of a communication in the form of a failure signal viathe interfaces E/A5.

In a subsequent step 604, the multiplexer 606 forms a substitute signalfor the failed wheel signal using the signal of at least one of thesensors S2 to S4, from which wheel signals are still received (which aretherefore respectively assigned to the right-hand rear wheel RR which isnot affected by the failure, right-hand front wheel FR and left-handrear wheel RL). Signals of other sensors which are installed in thevehicle can also be used to generate the substitute signal.

The substitute signal can be formed in different ways. In the presentexemplary embodiment according to FIG. 5, the formation of thesubstitute signal comprises making a selection among the wheel signalsof those sensors S2 to S4 which are assigned to the wheels RR, FR and RLwhich are not affected by the failure. Specifically, in the exampleaccording to FIG. 5 the wheel signal of the wheel sensor S2 which isassigned to the right-hand rear wheel RR is selected. The multiplexer406 subsequently copies the wheel signal which is supplied by the wheelsensor S2 to the output A1. In other words, the substitute signalcorresponds to the wheel signal which is supplied by the sensor S2 andis output at the output Al of the signal-processing device 402 as aregular wheel signal of the sensor S1.

In other implementations, the substitute signal can be based decisivelyon the selected wheel signal (here the wheel signal of the wheel sensorS2) but can deviate therefrom somewhat. Therefore, the multiplexer 406can mix, for example, a portion of one or more of the wheel signals ofthe wheel sensors S3 and S4 with the wheel signal of the wheel sensor S2in such a way that the resulting substitute signal is still decisivelybased on the wheel signal of the wheel sensor S2.

In a further step 606, the substitute signal can be fed via the outputA1 of the signal-processing device 402 to the input E1 of the ABS unit404. From the point of view of the ABS unit 404 the substitute signalwhich is obtained via the input El is a “normal” wheel signal of thewheel sensor S1 since it has been obtained via the input E1.

Accordingly, despite the wheel signal failure with respect to the sensorS1, the ABS unit 404 receives, at all four inputs E1 to E4, a wheelsignal which is assigned to that wheel which is in turn assigned to thecorresponding input E1 to E4. As explained above, the wheel signal whichis received at the input E1 is, however, a substitute signal for thewheel signal which is affected by the failure. The failure of the wheelsignal for the wheel which is assigned to the sensor S1 can becommunicated to the ABS unit 404 via the interfaces E/A5 (however, sucha communication can also be dispensed with).

On the basis of the wheel signals received via the inputs E1 to E4, theABS logic 408 carries out a slip detection and, if necessary, a slipcontrol. According to the example illustrated in FIG. 5, the ABS logic408 arrives at the conclusion that in each case a slip controlintervention is necessary at the wheel brakes 130B and 130A.Accordingly, corresponding actuation signals are output via the outputsA1 and A2 of the ABS unit 404.

According to the exemplary embodiment illustrated in FIG. 5, anindividual wheel sensor error can be compensated by suitablesubstitution with a wheel signal, still present, of another wheelsensor. In the simplest case, therefore, as illustrated in FIG. 5, aselected wheel signal is “copied” onto the wheel signal which isaffected by the failure so that the ABS unit 404 can continue to beoperated in an unchanged form or with only few adaptations. Of course,the substitute signal does not necessarily have to be a copy of one ofthe remaining wheel signals but rather it is also possible to carry outrelatively complex processing operations in the multiplexer 406 in orderto obtain the substitute signal. These further processing operations canbe based on a plurality of the wheel signals which are still presentand/or on additional sensor signals (such as for example a longitudinalacceleration, a transverse acceleration, a yaw rate, a steering angleand/or a main cylinder pressure). Such an additional sensor system canalso be used by the ABS logic 408 in order to improve, on the basis ofthe substitute signal, the ABS control behavior at the wheel which isaffected by the failure. With such a configuration it is then necessaryfor the signal-processing device 402 to inform the ABS unit 404 aboutthe wheel which is affected by the signal failure.

A coefficient of friction of the roadway is determined for each wheel bymeans of the wheel speeds or the yaw rate or both. In this way, inparticular different coefficients of friction of the roadway ondifferent sides of the vehicle can be detected (i.e. a split μ detectioncan be carried out). The intention is that despite the use of thesubstitute signal it will continue to be possible to take into accountdifferent coefficients of friction of the roadway, and therefore a splitμ situation, in conjunction with the ABS control. Exemplary selectionstrategies for an unknown or homogenous underlying surface, on the onehand, and in the case of a detected split μ situation, on the other,will now be explained in relation to FIGS. 7A to 7D, and respectivelyFIGS. 8A to 8D.

FIGS. 7A to 7D show wheel signal selection strategies in the case of ahomogenous underlying surface (that is to say the same coefficient offriction of the roadway on both sides of the vehicle) or in the case ofan unknown underlying surface (that is to say if, for example, fortechnical or other reasons, no split μ detection can be carried out).

In these cases, the wheel signals are essentially replaced for eachside. Therefore, if according to FIGS. 7A and 7B the wheel signal whichis assigned to a rear wheel is affected by the failure, the failed wheelsignal is generated on the basis of the wheel signal of the rear wheelwhich lies opposite the wheel affected by the failure.

Since the wheel signals for the wheels of the front axle are stillpresent, a split μ detection can be carried out on the basis of thesewheel signals. The split μ detection can be carried out either by thesignal-processing device 402 or by the ABS unit 404 or by both of thesecomponents independently. If the split μ detection is carried out by theABS unit 404, the result of this detection can be communicated to thesignal-processing device 402 via the interfaces E/A5. The selection ofthe wheel signals “to be coped” in relation to the generation ofsubstitute signals can then be made by the signal-processing device 402on the basis of this communication.

If signals are present from other sensors which point, for example, tothe yaw rate, the longitudinal acceleration, the transverse accelerationor the steering angle, this information can be additionally used for thesplit μ detection.

If as illustrated in FIGS. 7C and 7D, the wheel signal for a front wheelfails when the coefficient of friction of the roadway is unknown orhomogenous, the wheel signal of the front wheel which respectively liesopposite is also used to form the substitute signal (e.g. is copied).However, a conventional split μ situation can then no longer be detectedsince there is then only a single wheel signal available at the frontaxle. Therefore, the wheel diagonals, as also illustrated in FIGS. 7Cand 7D, are used for the split μ detection as a compensation for thefailed front wheel signal. Specifically, the split μ detection iscarried out on the basis of the wheel signals of sensors which areassigned to a front wheel not affected by a failure and a rear wheellying diagonally opposite this front wheel. Signals from further sensorscan also be used again here to improve the split μ detection.

If a split μ situation has been detected, the wheel signal selectionstrategy is correspondingly adapted, as illustrated in FIGS. 8A to 8D.

FIG. 8A illustrates the case in which a wheel signal for a rear wheel onthe side with the high coefficient of friction has failed. In this splitμ situation, the wheel signal from the rear wheel on the side of the lowcoefficient of friction is used to form the substitute signal. In an ABScontrol process a select low control strategy can be activated for therear axle, wherein this control strategy is actually intended forunknown or homogenous surfaces. According to the principle of the“select low”, the brake pressure at the two rear wheels is controlled ina corresponding way, wherein as a principle for the control, that rearwheel is used at which the slip is greatest (or generally which has thegreatest tendency to lock). In order to check the plausibility of thesplit μ situation further, it is also possible to have recourse to thewheel signals of the front axle. Plausibility checking is intended tomean here that it is continuously checked whether the split μ situationwhich has been previously detected persists. If this is no longer thecase, there can be recourse to one of the scenarios according to FIGS.7A to 7D.

FIG. 8B illustrates the case in which the wheel signal for a rear wheelon the side with the low coefficient of friction is affected by thefailure. In this case, the wheel signal of a front wheel on the sidewith the low coefficient of friction is used to form the substitutesignal. Further plausibility checking of the split μ situation can becarried out again by means of the wheel signals received for the frontaxle.

In the scenario according to FIG. 8C, the failure of a wheel signal fora front wheel on the side with the high coefficient of friction isassumed. In this case, the wheel signal for the wheel which would haveto apply the main braking torque has failed. The substitute signal isthen formed on the basis of the wheel signal of the rear wheel on thesame side. Furthermore, in order to obtain minimum deceleration at therear axle an individual control process is activated, insofar as it isnecessary (the brake pressure is therefore set on a wheel-specificbasis). In order to increase the braking capacity at the wheel which isaffected by the failure of the wheel signal, a yaw-rate-dependentstability control process can be carried out, at any rate for as long asa yaw rate signal is present. In this context, the maximum brakepressures on the side with the high coefficient of friction can besuitably limited so that an onset of slip is improbable. Furtherplausibility checking of the detected split μ situation can be carriedout by means of the diagonal indicated in FIG. 8C (front wheel on theside with a low coefficient of friction and rear wheel on the side witha high coefficient of friction).

Finally, FIG. 8D illustrates the failure of the wheel signal for a frontwheel on the side with a low coefficient of friction. In this case, thesubstitute signal for the front wheel is determined on the side with alow coefficient of friction by the wheel signal for the rear wheel onthe same side on the side with the low coefficient of friction. Thediagonal which is indicated in FIG. 8D, that is to say the wheel signalsfor the front wheel on the side with the high coefficient of frictionand the rear wheel on the side with the low coefficient of friction canthen be used again for further plausibility checking of the split μsituation.

In addition, the wheel selection strategies for forming the substitutesignal have to be mirrored if, in contrast to the situation indicated inFIGS. 8A to 8D, the right-hand side of the vehicle is the side with thehigh coefficient of friction and the left-hand side of the vehicle isthe side with the low coefficient of friction.

The precise coefficient of friction of the underlying surface of a wheelaffected by a wheel signal failure can always be estimated on the basisof the known coefficient of friction of the remaining three wheels. Forthis reason, it is appropriate, to be on the safe side, to reduce theprobability of unnoticed locking of the wheel which is affected by thefailure of the wheel signal. For this purpose, the increase in the brakepressure at the wheel for which a wheel signal failure has beendetermined can be suitably adapted. In particular, a brake pressurecontrol strategy can be provided for increasing the safety marginbetween the wheel with the known coefficient of friction on the basis ofwhich the substitute signal has been generated, and the wheel with theunknown coefficient of friction which is affected by the wheel signalfailure.

FIG. 9 illustrates in this context a flowchart 900 for a method whichcan be carried out by means of the ABS 400 according to FIG. 4 and, inparticular, by the ABS unit 404. In addition or as an alternative, thecorresponding method can also be implemented in an EBB or EBW controlunit (cf. for example reference symbol 310 in FIG. 2). In other words,the method is not limited to execution in conjunction with anABS-assisted braking operation.

The method starts in step 902 with the determination as to whether oneof the wheels is affected by a failure of the corresponding wheelsignal. If the method is carried out by the ABS unit 404, thedetermination can be carried out on the basis of a failure signal whichthe ABS unit 404 has received from the signal-processing device 402 viathe interfaces E/A5 (cf. the corresponding arrow in FIG. 5).

Furthermore, wheel signals for all the wheels are received in step 904,wherein the corresponding wheel signal for a wheel which is affected bya wheel signal failure is received in the form of a substitute signal.In the scenario illustrated in FIG. 5, a substitute signal is thereforereceived for the left-hand front wheel from the wheel sensor S2.

The steps 902 and 904 can be carried out in any desired sequence. Inother words, the step 904 could also precede the step 902, or the twosteps 902 and 904 could be carried out simultaneously.

In a further step 906, a target pressure for a wheel at which anincrease in brake pressure is necessary is calculated. The increase inbrake pressure can be carried out for a normal service brakingoperation, an ABS-assisted service braking operation or an emergencybraking operation (with or without ABS assistance). Specifically, theincrease in brake pressure takes place in accordance with the wheelsignal which is obtained for this wheel, on the one hand, and inaccordance with the determination as to whether this wheel is affectedby a wheel signal failure, on the other. In this context, if the wheelat which the increase in brake pressure is necessary is affected by thewheel signal failure, a lower target brake pressure can be provided thanif this wheel were not affected by the wheel signal failure.

For example, a prescribed pressure difference between the wheel forwhich a wheel signal failure was determined and the wheel whose wheelsignal forms the basis for the calculation of the substitute signal(“comparison wheel”) can be set. The pressure difference can be providedin such a way that the wheel braking torque at the wheel which isaffected by the wheel signal failure always remains a little lower thanthe wheel braking torque at the comparison wheel. In this way, a safetymargin is introduced which makes it less probable that the wheelaffected by the wheel signal failure will lock without being noticed.The pressure difference can be set in such a way that the furtherincrease in pressure at the wheel affected by the wheel signal failureis set in a suitable way to set the target pressure only starting from acertain vehicle deceleration or a certain wheel brake pressure.

A pressure difference which results from the brake pressure differenceat the wheels which are assigned to a specific vehicle axle brings abouta difference in wheel braking torque at these wheels, as a result ofwhich the vehicle could pull on one side. The degree of this pulling onone side depends on the difference in wheel braking torque. Anappropriate variable for the difference in wheel torque, and thereforethe difference in hydraulic pressure, can be made dependent on theavailability of other sensors (for example of a yaw rate sensor, of atransverse acceleration sensor, etc.). If the corresponding signals fromone or more other sensors (in addition to the wheel signals) arepresent, a greater difference in brake pressure can be set. Pulling ofthe vehicle to one side, which occurs in this context, can also possiblybe pilot controlled by known strategies such as straight line braking(SLB), at the axle which is not affected by the wheel signal failure. Onthe other hand, if the further sensor signals are not available, acorrespondingly smaller difference in hydraulic pressure can be set inorder to be able to make the situation easier to cope with.

In order to increase the safety margin in the case of an ABS-assistedbraking operation, the slip threshold of the wheel affected by the wheelsignal failure can, to be on the safe side, be reduced by a certainabsolute amount or factor in comparison with the wheel on the same axle.This procedure also makes possible in respect of the trend a somewhatlower braking torque at the wheel which is affected by the wheel signalfailure.

FIG. 10 shows in a schematic diagram the setting of a difference inhydraulic pressure for a service braking operation before and during anABS assistance operation. It is assumed here that the right-hand frontwheel is affected by the wheel signal failure.

In the upper region of the graphic, four lines are shown which areoffset parallel to one another and illustrate the continuously fallingvehicle speed. Furthermore, for each of these four lines the wheel speedwhich has been calculated from the wheel signal of the correspondingwheel sensor S1 to S4 is illustrated. Owing to the failure of the sensorS3 for the right-hand front wheel, the failed wheel signal has beenreplaced by the wheel signal of the sensor S1 for the left-hand frontwheel (cf. arrow top left).

Continuous deviations between the vehicle speed and the respective wheelspeed can be clearly seen. If such a deviation exceeds a slip threshold,an ABS assisted process of the service brake operation is carried out asillustrated on the right-hand side of the graphic. The slip calculationis based on the calculation of a deviation of an individual wheel speedfrom the vehicle speed. The vehicle speed can be determined on the basisof the wheel speed of a slip-free wheel or in some other way (e.g. onthe basis of a satellite-based positioning system).

In the lower part of FIG. 10, the increase in brake pressure at the fourwheel brakes 130A to 130D is illustrated. Overall, a higher brakepressure is built up at the two front wheels (FL, FR) than at the tworear wheels (RL, RR). The brake pressure at the two front wheels is sethere in such a way that a pressure difference is set. Specifically, thebrake pressure at the right-hand front wheel, which is affected by thewheel signal failure, is always lower by a certain “pressure delta” thanthe brake pressure at the left-hand front wheel which is not affected bythe wheel signal failure. This applies both to the normal servicebraking operation and to the ABS-assisted service braking operation. Thesetpoint brake pressure at the front wheels, which is requested, forexample, by a system for autonomous or partially autonomous driving orvia driver, is also indicated in FIG. 10. The driver's request can bedetermined from the main cylinder pressure.

Particularly the pressure delta before the ABS assistance process canalso be applied taking into account a reduction in noise and/orvibrations. In this way, the noise vibration harshness (NVH) propertiesof the brake system 100 can be improved. The rear axle in FIG. 10 iscontrolled, for example, according to the principle of dynamic rearproportioning (DRP) (and not yet in the abovementioned SLB mode).

As is apparent from the exemplary embodiments, the solution proposedhere permits a higher level of availability of the vehicle brake system,and in particular of the ABS, in the case of a wheel signal failure.This higher availability is indispensable, in particular, for autonomousor partially autonomous driving, but is also desirable in conventionalvehicles.

1-20. (canceled)
 21. A signal-processing device (402) for a vehiclehaving an ABS unit (404) and a plurality of wheels, which are eachassigned a sensor (S) for generating wheel signals, wherein the device(402) is designed: to detect (602) a failure of a wheel signal; to form(604) a substitute signal for the failed wheel signal using the wheelsignal of at least one sensor (S) which is assigned to a wheel which isnot affected by the failure; and to feed (606) the substitute signal tothe ABS unit (404).
 22. The device as claimed in claim 21, designed tomake a selection from wheel signals of those sensors (S) which areassigned to wheels which are not affected by the failure, wherein thesubstitute signal corresponds to the selected wheel signal or isdecisively based thereon.
 23. The device as claimed in claim 22,designed to carry out the selection in accordance with a detected splitμ situation.
 24. The device as claimed in claim 21, wherein the failedwheel signal is assigned to a rear wheel (RR, RL), and the device isdesigned to carry out split μ detection or split μ plausibility checkingon the basis of wheel signals of sensors (S) which are assigned to frontwheels (FR, FL); or wherein the failed wheel signal is assigned to afirst front wheel (FR; FL), and the device is designed to carry outsplit μ detection or split μ plausibility checking on the basis of wheelsignals of sensors (S) which are assigned to a second front wheel (FL;FR) which is not affected by the failure and to a rear wheel (RR; RL)which is located diagonally opposite the second front wheel (FL; FR).25. The device as claimed in claim 22, designed to select the wheelsignal for the wheel which, on the vehicle, lies opposite the wheelwhich is affected by the failure.
 26. The device as claimed in claim 25,designed to carry out the selection of the wheel signal for the wheelwhich, on the vehicle side, lies opposite the wheel which is affected bythe failure, when a split μ situation is not present or is not detected.27. The device as claimed in claim 22, designed to select, in the caseof a detected split μ situation and in the case of a failure of a wheelsignal for a first rear wheel (RR; RL) on a side with a high coefficientof friction, the wheel signal for a second wheel (RL; RR) on a side witha low coefficient of friction; or in the case of a failure of a wheelsignal for a rear wheel (RL, RR) on a side with a low coefficient offriction, the wheel signal for a front wheel (FL, FR) on the side withthe low coefficient of friction; or in the case of a failure of a wheelsignal for a front wheel (FL, FR) on a side with a high coefficient offriction, the wheel signal for a rear wheel (RL, RR) on the side with ahigh coefficient of friction; or in the case of a failure of a wheelsignal for a first front wheel (FL; FR) on a side with a low coefficientof friction, the wheel signal for a second front wheel (FR; FL) on theside with a low coefficient of friction.
 28. Anti-lock brake system(400) comprising: the signal-processing device (402) as claimed in claim21; and an ABS unit (404).
 29. A system as claimed in claim 28, whereinthe ABS unit (404) comprises an assigned wheel signal input (E1-E4) foreach sensor (S1-S4); and the signal-processing device (402) is designedto feed the substitute signal to that wheel signal input (E1-E4) whichis assigned to the failed wheel signal.
 30. A system as claimed in claim28, wherein the signal-processing device (402) is designed to generate afailure signal, which indicates the wheel which is affected by thefailure; and the ABS unit (404) has an input (E/A5) for the failuresignal.
 31. The system as claimed in claim 28, wherein the ABS unit(404) is designed to carry out at least one of the following steps onthe basis of at least the substitute signal detecting a need for ananti-lock brake control operation at the wheel affected by the failure;and carrying out an anti-lock brake control operation at the wheelaffected by the failure.
 32. The system as claimed in claim 28, whereinthe signal-processing device (402) is designed to make a selection fromwheel signals of those sensors (S) which are assigned to wheels whichare not affected by the failure, wherein the substitute signalcorresponds to the selected wheel signal or is decisively based thereonand designed to select, in the case of a detected split μ situation andin the case of a failure of a wheel signal for a first rear wheel (RR;RL) on a side with a high coefficient of friction, the wheel signal fora second wheel (RL; RR) on a side with a low coefficient of friction; orin the case of a failure of a wheel signal for a rear wheel (RL, RR) ona side with a low coefficient of friction, the wheel signal for a frontwheel (FL, FR) on the side with the low coefficient of friction; or inthe case of a failure of a wheel signal for a front wheel (FL, FR) on aside with a high coefficient of friction, the wheel signal for a rearwheel (RL, RR) on the side with a high coefficient of friction; or inthe case of a failure of a wheel signal for a first front wheel (FL; FR)on a side with a low coefficient of friction, the wheel signal for asecond front wheel (FR; FL) on the side with a low coefficient offriction; and the ABS unit (404) being designed to carry out at leastone of the following steps: carrying out a select low closed-loopcontrol operation of the rear axle in the case of a failure of a wheelsignal for a rear wheel (R, RR); and activating an individualclosed-loop control operation in order to bring about minimumdeceleration at the rear axle in the case of failure of a wheel signalfor a front wheel (FL, FR).
 33. The system as claimed in claim 28,wherein the failed wheel signal is assigned to a rear wheel (RL, RR),and the system is designed to carry out split μ detection or split μplausibility checking on the basis of wheel signals of sensors (S) whichare assigned to front wheels (FL, FR); or wherein the failed wheelsignal is assigned to a first front wheel (FL; FR), and the system isdesigned to carry out split μ detection or split μ plausibility checkingon the basis of wheel signals of sensors (S) which are assigned to asecond front wheel (FR; FL) which is not affected by the failure and toa rear wheel (RL; RR) which lies diagonally opposite the second frontwheel (FR; FL).
 34. The system as claimed in claim 33, wherein the ABSunit (404) is designed to generate an indication signal which indicatesthe split μ situation; and the signal-processing device (402) has aninput (E/A5) for the indication signal.
 35. The system as claimed inclaim 28, designed to calculate a target brake pressure for a wheel atwhich an increase in brake pressure is necessary, in accordance with thewheel signal obtained for this wheel and in accordance with adetermination as to whether this wheel is affected by a wheel signalfailure.
 36. The system as claimed in claim 28, wherein the ABS unit(404) is designed to calculate, for the wheel affected by the failure, aslip threshold for the use of an anti-lock closed-loop controloperation, wherein the calculated slip threshold is lower than if thewheel were not affected by the failure.
 37. A vehicle designed forautonomous or partially autonomous driving comprising a system (400) asin claim
 28. 38. A signal-processing method for a vehicle having an ABSunit (404) and a plurality of wheels which are each assigned a sensor(S) for generating wheel signals, wherein the method comprises:detecting (602) a failure of a wheel signal; forming (604) a substitutesignal for the failed wheel signal using the wheel signal of at leastone sensor (S) which is assigned to a wheel which is not affected by thefailure; and feeding (606) the substitute signal to the ABS unit.
 39. Acomputer program with program code for carrying out the method asclaimed in claim 38 when the computer program is run on a processor unit(302A, 304A).
 40. A control unit (302, 304) or control unit system (300)having a processor unit (302A, 304A) and a memory (302B, 304B) whichcontains the computer program as claimed in claim 39.